Damping of mechanical vibrations, as opposed to isolation of vibrations, may be desirable in some applications to reduce dynamic motions. Usually, damping is required in systems which are load controlled, i.e., where loads applied to the system cause dynamic oscillatory motions. Adding large amounts of hysteretic damping to these systems can reduce such motions. One application where high damping may be required is in the so-called hingeless or flexbeam rotor system of a helicopter. Typically, these composite rotor systems do not exhibit sufficient internal damping to damp lead-lag motions. As such, a lead-lag damper may be required to provide supplemental damping to the system. Prior art systems have greatly increased system damping by the addition of passive elastomeric dampers with resulting reductions in rotor blade lead-lag motions.
One such passive elastomeric damper is described in U.S. Pat. No. 4,778,343 to Hahn et al., the disclosure of which is hereby incorporated by reference herein. The Hahn '343 patent describes a composite rotor system which utilizes dual elastomer dampers 9 to damp lead-lag motions. At the same time, these dampers 9 also react to flapping loads. The dampers are generally used in opposed pairs, one on the top of the pitch case (otherwise referred to as the cuff) and one on the bottom thereof. An elastomeric bearing 8 interconnects the dampers 9 to the blade root 2.1. These passive dampers 9 are bonded in highly damped polymers (loss factors of 0.5 or more). The highly damped material hysteretically damps the motion of the blades by converting motion into heat.
U.S. Pat. No. 5,092,738 to Byrnes et al., which is hereby incorporated by reference herein, describes another passive damper system including damper units 52 for a hingeless or flexbeam rotor system. The Byrnes et al. '738 patent describes the use of a spherical elastomeric bearing 64 to perform a centering function, accommodate torsional and cocking motions, and react to flapping loads.
U.S. Pat. No. 4,893,988 to Sato teaches yet another passive damper system including an elastomeric pivot 7 interconnected to elastomeric dampers 9 to a flex beam 2. The blade 4 and pitch sleeve 3 attach to the outboard end of the flex beam 2.
An elastomeric lead-lag damper for a more conventional helicopter rotor system is described in the commonly assigned U.S. Pat. No. 3,758,230 to Potter.
As an improvement over passive elastomer dampers, damper devices have been developed which utilize combinations of fluid and elastomer to increase damping levels and linearity achievable by passive elastomer dampers. U.S. Pat. No. 5,374,039 to Schmidt et al., which is incorporated by reference herein, describes a fluid and elastomer damper 20 for use on an articulated rotor system. The damper utilizes a fluid contained within the damper which is throttled back and forth between opposed chambers 26, 28 and through fluid passageways 68 to create additional damping over and above what is available from elastomeric dampers (hysteresis) alone. These dampers 20 will be referred to herein as throttled-type dampers. The damper further includes means for limiting the dynamic pressure buildup therein.
These throttled-type dampers produce fluid damping in addition to the hysteretic damping. However, although these throttled-type dampers provide excellent properties, a simpler construction may be needed in some applications. Furthermore, providing multiple fluid cavities can require volume that may not be available in some applications. This is especially true in helicopter rotor applications, in which any increase in the size of the damper can mean more exposure to the air stream and, thus, an increase in the drag on the rotor blade system. In addition, without proper sizing of the passages and proper viscosity, throttled-type dampers tend to be somewhat nonlinear.
A fluid and elastomer damper for a more conventional helicopter rotor system is described in U.S. Pat. No. 4,566,677 to LePierres where fluid passes between chambers 23, 24 through working passages 25.
The prior art also describes devices which utilize an inner member or piston for moving through a fluid, as opposed to throttling, to provide additional nonhysteretic damping (referred to herein as piston type dampers). U.S. Pat. No. 3,874,646 to Vernier, U.S. Pat. No. 3,154,273 to Paulsen, U.S. Pat. No. 3,141,523 to Dickie, U.S. Pat. No. 4,790,521 to Ide et al., U.S. Pat. No. 4,779,585 to Behrens et al., and U.S. Pat. No. 4,770,396 Jouade, describe piston type devices in which a piston is used in a closed cavity which includes a viscous fluid. Movement of the piston within the cavity causes a stirring of the fluid, thus increasing damping over and above that available from the elastomer alone.
U.S. Pat. No. 5,540,549 to McGuire teaches a piston type damper with particular applicability to the hingeless rotor system. The damper 20 includes a piston 38 moveable in a cavity 34 full of viscous fluid 36. The piston moves within the cavity 34 as a result of relative movement between the inner member 22 and the outer member 24, thereby causing the fluid 36 to flow about the piston 38 and through a flow area. The damping force created comprises a throttling component from the fluid flowing through the flow area and a viscous drag component developed from the fluid acting on a surface area of the piston. Notably, the piston 38 is rigidly connected to the inner member 22, thus the gaps must be set wide enough to accommodate static motions due to flexing of the flexible section 28. That is, the adjacent walls of the outer member 24 are displaced with respect to the side edges of the piston 38 whereby the areas of the passages (gaps) defined therebetween are altered. This can detract from the effectiveness of the damper in that the gaps must be set wider than desired to accommodate such static motions.
Another problem presented by the afore described design relates to the sizes of the gaps or passages. Smaller passages allow more damping to be generated for a particular fluid of a given viscosity. Generally, more viscous fluids must be employed to obtain the same damping from larger gaps. It is desirable that the fluid be of relatively low viscosity because lower viscosity fluids tend to maintain a more uniform viscosity over a wider range of operating temperatures. On the other hand, the damper must accommodate an anticipated amount of lateral deflection of the elastomer section. The passages must be large enough to allow the damper to move through its full range of deflection without causing contact between the various components therein. Thus, in order to prevent interference between the components, it may be necessary to provide fluid passageways which are larger than would otherwise be desired.
Another problem which may be experienced with dampers for attaching between a rotor cuff and a rotor flexure (in which the damping devices of the type have an elastomer section comprised of laminated shims and elastomer layers) is that the bond between an elastomer layer and an attached shim may fail. When this occurs, for example, under high pitch conditions, the clastomer section may separate from the inner member, whereupon the pivot point between the cuff and the flexure may be lost.